It is well known in the prior art that antennas for radiating and receiving radio signals may be formed from several individual antenna elements. By arranging the antenna elements with specific geometry, and combining signals associated with the individual elements with a specific phase and amplitude relationship, the individual elements cooperate to form a unitary antenna structure.
Each of the individual antenna elements in such an antenna (in a transmit application) radiates a signal which is common in frequency, but altered in amplitude and phase from the other elements. As a result, the individual signals combine in space at varying phase and amplitude levels to create an antenna pattern. The signal combination essentially follows a three dimensional vector addition function. The combination of signals which are in phase results in signal lobes. The cancellation of signals which are completely out of phase (i.e., 180.degree.) results in signal nulls. For all phase angles in between these extremes, partial cancellation occurs which shapes the signal lobes. The resultant signal is referred to as the antenna pattern. The antenna pattern is characterized by the number of lobes, thc magnitude of the lobes (gain), the direction of the lobes and the relative magnitude of the lobes in differing directions (directivity).
In multi-element array antennas, the gain, directivity and lobe direction may be varied by controlling the phase of the signals driving the individual elements. This type of antenna is conventionally referred to as a phased array. An in depth treatment of conventional phased arrays is presented in The Radar Handbook, Second Edition, edited by Merrill Skolnik, published in 1990 by McGraw-Hill, which is incorporated herein by reference.
Phased arrays may be formed as linear arrays (FIG. 1), planar arrays (FIG. 2), or conforming arrays (FIG. 3). The linear array shown in FIG. 1 is capable of producing an antenna pattern which can be rotated along (scanned) a two dimensional plane by varying the phase of the signals driving each of the antenna elements 2. The planar and conforming arrays are capable of scanning in three dimensional space by appropriately driving the individual antenna elements 2.
Regardless of the chosen array geometry, the signal along each path between a signal source and the antenna elements have a controlled phase relationship in order to form a desired antenna pattern. This is achieved by controlling signal power division ratios and the phase shift in the electrical transmission path between the signal source and each antenna element. A structure which performs this function is generally referred to as an antenna feed or processor.
FIG. 4 illustrates a conventional "corporate feed" antenna feed topology. In a corporate feed, a signal source 4 simultaneously drives, in parallel, each of the antenna elements 2. In a corporate feed, the length of each transmission line segment 6 is the same for each antenna element 2. The phase of the signal driving each element is controlled by an analog phase shift network 8. For a variable antenna pattern, each antenna element 2 will have an individually controllable phase shift network 8.
An alternative antenna feed network, a series feed, is illustrated in FIG. 5. In the conventional series feed network, a series of antenna elements 2 are connected in a single transmission line 6 with a built in phase progression between the antenna elements 2. The phase progression is determined in part by the length of the transmission line 6 (physical path length) between successive antenna elements 2. The phase of the signal at each element 2 is related to the electrical path length between antenna elements 2. The electrical path length, expressed in wavelengths, changes with frequency for a fixed physical path length. Therefore, the phase progression between antenna elements 2 in a series feed varies with frequency. For variable antenna patterns, variable analog phase shift networks 8 may be inserted between the antenna elements 2.
A third conventional antenna feed network, a space feed network, is illustrated in FIG. 6. In the space feed network, a source antenna 10 is electrically connected to a signal source 4. The source antenna 10 radiates a signal received from the signal source 4. The radiated signal is received by a series of pickup elements 12. The received signals are then coupled through phase and amplitude shift networks 9 to the antenna elements 2 for transmission.
The antenna feed topologies illustrated in FIGS. 4, 5 and 6 each require the use of phase shift networks or time delay devices in line with each antenna element to achieve dynamic antenna pattern control or scanning. Thus, in a large two-dimensional array antenna of M*N elements, M*N phase shift networks are required.